Last time I checked, Kirby was the only Nintendo franchise to be heavily experimented with. Sure a lot of elements carry over, but to call it as stale as the Mario series is pretty much an insult.
Stale is not a word I used, and I'm not sure what word would be the right word. Backdrop? Using the same backdrop gets to be a bit blah. (and I absolutely love/adore Canvas Curse, which was my first proper Kirby, its when I then went and played other Kirby's that I noticed the problem, to me, of course.)
edit: It's different for Mario Kart for example because although every title has to have a Rainbow Road/Bowser Castle/Mario Raceway, they're all stylistically different enough to feel fresh/new each time. (haven't played 7 yet though) Also see: Sonic 16bit series zones from title to title.